But aca­demics at the Univer­sity of Sh­effield now be­lieve they have found out how Planet 9 ended up on the out­skirts of our so­lar sys­tem. It has pre­vi­ously been sug­gested that the planet ei­ther moved to the edge of the so­lar sys­tem from the in­ner re­gions, or it was cap­tured when the Sun was still in its birth star clus­ter.

The Sh­effield team has found the cap­ture sce­nario is ‘un­likely’ and Planet 9 must have formed around the sun. They say it prob­a­bly formed closer to home than pre­vi­ously thought.

A team led by Dr Richard Parker from the univer­sity’s Depart­ment of Physics and Astron­omy has found that Planet 9 is “un­likely” to have been cap­tured from an­other plan­e­tary sys­tem, as has pre­vi­ously been sug­gested, and must have formed around the sun. Planet 9 is at least 10 times big­ger than Earth, mak­ing it un­likely that it formed at such a large dis­tance from the sun.

Dr Parker said: “We know that plan­e­tary sys­tems form at the same time as stars, and when stars are very young they are usu­ally found in groups where in­ter­ac­tions be­tween stel­lar sib­lings are com­mon. There­fore, the en­vi­ron­ment where stars form di­rectly af­fects plan­e­tary sys­tems like our own, and is usu­ally so densely pop­u­lated that stars can cap­ture other stars or plan­ets.”